Aligning the Frame: How Reference Frame Differences Impact Markerless vs. Marker-Based Comparisons

Summary

When comparing markerless and marker-based motion capture, researchers often find differences in the kinematic outputs. But how much of these discrepancies are true differences in how the movement is measured, and how much comes down to how each system defines the anatomical reference frames? A new study out of Rush University Medical Center and Aesculap AG explores this question using Theia3D - and finds that aligning coordinate systems dramatically improves agreement between systems.

Research Overview

The study recruited 10 healthy participants who performed walking trials which were simultaneously recorded by Qualisys marker-based and markerless (video) cameras. Markerless video data was processed using Theia3D. The researchers applied a novel post-processing method called REFRAME to optimize the alignment of the local coordinate systems between the two motion capture systems. Their goal: determine if observed differences in knee joint kinematics were primarily due to inconsistencies in reference frame orientation, or fundamental differences in the underlying motion data captured by each system.

Key Findings

Before REFRAME was applied, the markerless and marker-based systems showed noticeable differences, particularly in the frontal and transverse planes of motion, consistent with previous literature comparing Theia3D and marker-based systems. After REFRAME alignment:

Root-mean-square error (RMSE) between systems dropped from 3.9° to 1.7° for flexion/extension, 6.1° to 1.7° for ab/adduction and from 10.2° to 2.5° for internal/external rotation.
The percentage of the gait cycle with statistically significant differences decreased by ~50% across all three planes.
Inter-trial variability in Theia3D kinematics decreased, showing improved consistency after REFRAME was applied.

Implications for Researchers and Clinicians

This study shows that even small discrepancies in coordinate system definitions can lead to large differences in reported joint angles – particularly in frontal and transverse plane angles. For researchers and clinicians comparing systems or tracking changes over time, understanding the impact of reference frame alignment is crucial. This work also provides the necessary context for interpreting differences reported in any previous or future comparisons between motion capture systems, and highlights the value of methods like REFRAME to clarify the source of differences in these comparisons.

The Role of Theia3D in Movement Analysis

Theia3D’s markerless approach offers streamlined setup, natural movement capture, and increased accessibility compared to traditional systems. While differences in coordinate systems can influence results, this study confirms that Theia3D captures fundamentally similar movement waveforms as gold-standard marker-based systems. When aligned appropriately, Theia3D’s outputs closely match those of traditional systems – reinforcing its utility for both research and clinical applications.

Future Research Directions

While this investigation focused on the knee joint, future studies should explore the hip and ankle to fully understand the impact of reference frame alignment across the lower limb. There’s also an opportunity to further validate REFRAME as a tool for harmonizing data across different motion capture technologies - supporting broader interoperability and cross-study comparisons.

Full Study & Further Reading

Read the full study in the Journal of Biomechanics.

To learn how Theia can support your research, book a demo.

‍

Recent Posts

How Tonal Uses Markerless Motion Capture from Theia3D to Scale Biomechanics Research and Personalize Strength Training

Tonal’s biomechanics team uses Theia3D markerless motion capture to accelerate research, validate motion models, and power features like Smart View. Learn how they’ve streamlined data collection, scaled biomechanics across participants, and turned lab insights into real-time, personalized strength training.

Improving Change of Direction Performance in Adolescent Soccer Players Using Markerless Motion Capture

A recent study published in Frontiers in Sports and Active Living highlights how brief technique modification training can significantly improve change of direction (COD) performance in adolescent soccer players. Using a markerless motion capture system, the study demonstrated measurable improvements in COD mechanics. This research underscores the value of markerless technology like Theia3D for coaches and sports scientists aiming to optimize performance and reduce injury in adolescent athletes.

Theia3D Now Supports NVIDIA GeForce RTX 50 Series Graphics Cards

Following our first major software update of 2025 - Theia Axiom - Theia3D now supports the newest generation of graphics cards from NVIDIA - the GeForce RTX 50 Series. These new graphics cards deliver significant performance improvements, allowing you to further push the limits of your markerless motion capture workflow with Theia3D. In this post, we break down what this update means for processing speeds and productivity.